S ilencing of SCD and SREBP1 Genes by Short Hairpin RNAs in Chicken Hepatic Cells in Vitro

RNA interference by short hairpin RNAs (shRNAs) is a widely used post transcriptional silencing mechanism for suppressing expression of the target gene. In the current study, ve shRNA molecules each against SCD and SREBP1 genes involved in denovo lipid biosynthesis were designed upon considering parameters such as secondary structures of shRNAs, mRNA target regions, GC content and thermodynamic properties (ΔG overall, ΔG duplex and ΔG break-target), synthesized and cloned in pENTR/U6 entry vector to knockdown the expression of SCD and SREBP1 genes. After transfection of these shRNA constructs into the chicken embryonic hepatocytes, expressions of the target genes were monitored by real time PCR. Signicant reduction (P<0.05) in the expression of SCD and SREBP1 genes was observed in hepatocytes. The shRNAs against SCD gene showed the knock down eciency ranged from 20.4% (shRNA5) to 74.2% (shRNA2). In case of SREBP1 gene, the shRNAs showed knock-down eciency ranging from 26.8% (shRNA4) to 95.85% (shRNA1). The shRNAs against both the genes introduced in chicken hepatocyte cells did not show any signicant impact on expression of immune response genes (IFNA and IFNB) in those cells. These results clearly demonstrated the successful down regulation of the expression of SCD and SREBP1 genes by the shRNA molecules against both the target genes under in vitro condition. It is concluded that the shRNA molecules against SCD and SREBP1 genes showed great potential to silence the expression of these genes under in vitro chicken embryonic hepatocyte cells. biosynthesis has benets like production of lean meat and egg and reduction of fat deposition in several organs of the birds. Silencing of target genes which are involved in the fat biosynthesis by using RNA interference in chicken primary hepatocyte culture helped in the devising suitable in vitro models for knock down of the target gene. This technique can be used further for production of the knock down chicken with reduced fat production. Till date, there were no reports available in the literature regarding the silencing of fatty acid synthesis genes in chicken. In the present study, the shRNA molecules were successfully transfected into the chicken hepatic cells and signicant down regulation of the target genes were observed. These results suggest that the chicken embryo hepatocytes can be used for in vitro model for various functional studies. Signicant reduction in the expression of SCD and SREBP1 genes in hepatocytes was observed after transfecting the shRNA molecules into them. The shRNA constructs against SCD gene showed the knock down eciency ranged from 20.4–74.2%. In case of shRNA constructs against SREBP1 gene, they showed knock down eciency ranging from 26.8–95.8%. The reasons behind the high knock down eciency of shRNA2 against SCD gene and shRNA1 molecule against SREBP1 gene might be lack of secondary structures in their anti-sense/guide strand, desirable GC percentage (43%-52%) in sense strand, more accessibility in the target mRNA region i.e stem loop structures in secondary structure of mRNA region and possessing thermodynamic properties falling in desirable range. The expression of the SCD and SREBP1 examined in the cell lysates showed lower expression in the cells transfected with shRNAs indicating the similar trends as of mRNA expression in the cell culture. Thus, these results clearly demonstrated the successful down regulation of the gene expression by designed shRNA molecules against both the target genes under in vitro condition. In previous reports, by using shRNA molecules in Myostatin gene [50], ACTRIIA gene [51] and ACTRIIB gene [32] knock down eciency of 68%, 87% and 82% were found respectively in chicken embryo broblasts cells. In duck embryo broblasts, different shRNA


Introduction
Genetic selection has greatly improved chicken production over the last 50 years which provides high quality dietary protein to mitigate our protein demand.
However, intensive genetic selection for rapid growth inadvertently selected for increased carcass fat. Carcass fat in broilers at 6 weeks of age accounts for 10-15% of the total carcass weight and deposits at a rate of up to 6 g fat/kg body wt/day between 42 and 49 days of age [1]. The predominant site of adipose deposition in chickens is abdominal fat and, to a less extent, subcutaneous fat and intramuscular fat [2]. This is probably because abdominal fat is proportionately the largest when body weights and growth rates are the critical traits under selection. Excess fat is an economic loss to the broiler industry by reducing the conversion of feed to meat, and the disposal of fat leads to additional economic loss. Excess fat may also contribute to the development of other detrimental traits, such as reduced reproductivity and immune competence, which consequently affect growth and yield of meat [3].
Fatty acids in animals can come from diet or be synthesized from glucose in lipogenic tissues such as liver and adipose tissues catalyzed by fatty acid synthase. Dietary fatty acids are absorbed by enterocytes, packaged with cholesterol, lipoproteins and other lipids into chylomicrons, and transported into circulation. Lipids synthesized in the liver are packaged into LDLs and released into the blood directly. TAG-rich VLDL are taken up by adipocytes and deposited into adipose tissue. In chickens, the contribution of portomicrons (similar to chylomicrons in mammals) from dietary fats to triglyceride is low, since there is only 5% of dietary lipids in regular feed [4]. Triglyceride (TAG) deposits in chicken adipose tissue mainly through the uptake of fatty acids from circulating TAG-rich VLDL synthesized by the liver. Increased VLDL levels were found virtually in chickens associated with high fatness, re ecting the critical role of the liver in fatness of chickens. The contribution of de novo lipogenesis in liver to fat deposition was also con rmed in chicken [5]. Only 15% of de novo lipogenesis occur in adipose tissue, and the rest is happened in liver [6]. The product of de novo lipogenesis in liver are secreted in the form of VLDL and delivered to other tissues. However, the basic step in the synthesis of lipids is the formation of saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs), the major fatty acid species in mammalian cell lipids.
One key regulator of the fatty acid composition of cellular lipids are Stearoyl-CoA desaturases (SCD), the endoplasmic reticulum-resident enzyme that catalyze the introduction of the rst double bond in the cis-delta-9 position of several saturated fatty acyl-CoAs, principally palmitoyl-CoA and stearoyl-CoA, to yield palmitoleoyl-and oleoyl-CoA, respectively [7,8]. Stearoyl-CoA desaturase (SCD) is an iron-containing enzyme that catalyzes a rate-limiting step in synthesis of the unsaturated fatty acids. The principal product of SCD is oleic acid, which is formed by desaturation of stearic acid.
Sterol regulatory element binding protein-1 (SREBP1) is a transcription factor that binds to a sequence in the promoter of a group of genes, called sterol regulatory element-1 (SRE1). This element is a decamer anking the LDL receptor gene and other genes involved in sterol biosynthesis. The SREBP1 regulates genes required for glucose metabolism and fatty acid and lipid production and its expression is regulated by insulin [9]. The SREBP1 regulates genes related to lipid and cholesterol production and its activity is regulated by sterol levels in the cell [10,11]. The transcription factor SREBP1 regulates de novo lipogenesis in the liver in response to increases in insulin. SREBPs are transcription factors of the basic helix-loop-helix leucine zipper family that are synthesized as precursors and bound to the endoplasmic reticulum membrane. In the present study, we have developed an in vitro model for reduction of SCD and SREBP1 expression in chicken embryonic hepatic cells by the short hairpin RNA molecules.

Experimental animals
The study was conducted in the control broiler chicken line maintained at ICAR-Directorate of Poultry Research, Rajendranagar, Hyderabad, India. The fertile eggs were collected from control broiler chicken and incubated at 98 o F and 85% relative humidity in the incubator for 12 to 13 days. The embryonated eggs of 12-13 days old were taken out for preparation of embryonic hepatocyte primary cell culture. The whole study was approved by the Institute Animal Ethics Committee (IAEC) and Institute Bio-safety Committee (IBSC) of ICAR-Directorate of Poultry Research, Rajendranagar, Hyderabad, India to carry out the animal experiment.
Designing of shRNA molecules: A total of 5 shRNA molecules each were designed from ORF of SCD (NCBI Accession No. NC_006093) and SREBP1 (NCBI Accession No. NC_006101) genes using BLOCK-iT RNAi Designer (https://rnaidesigner.thermo sher.com/rnaiexpress/) software (Table 1). To determine e ciency of shRNA molecules, secondary structures of shRNAs were predicted by in silico analysis in the web server of Mfold 2.3 version [12] (http://mfold.rna.albany.edu/?q=mfold/RNA-Folding-Form) was used after considering the all the required criteria. Secondary structures of antisense strands of shRNA molecules against SCD and SREBP1 genes were predicted by using the RNA fold program of the Vienna RNA web service version 2.0 [13] (http://rna.tbi.univie.ac.at/cgibin/RNAWebSuite/RNAfold.cgi). Further, Thermodynamic properties that governs the binding a nity of shRNA molecules and its target region on mRNA molecules [15] were predicted by Oligowalk program [14] in the RNA structure version 5.3 (http://rna.urmc.rochester.edu/cgibin/server_exe/oligowalk/oligowalk_form.cgi). Following thermodynamic properties [16] were included in the present study: i. ΔG overall: The net energy (ΔG in kcal/mol) resulting due to binding of target site by oligos, after consideration of all energy contributions viz. oligo-selfstructure energy and target structure breaking energy. The higher negative value of ΔG signi es stronger binding.
ii. ΔG duplex: It determines the stability of the duplex formed between guide strand of siRNA and the nucleotides at the target site. A less negative value of ΔG duplex denotes less stability of duplex and vice versa.
iii. ΔG break−target (disruption energy): The energy cost for disrupting local secondary structure at the mRNA target region. A more negative value denotes that the binding site is not completely open and less accessible.

Cloning of shRNAs:
A nal concentration of 50 μM double stranded oligos were prepared by mixing top strand DNA oligo (200 μM) -5 μl, Bottom strand DNA oligo (200 μM) -5μl, 10X Oligo annealing buffer -2μl and DNAse/RNase free water -8μl, by using the conditions 95°C for 4 minutes and then allowed to cool at room temperature for 5-10 minutes to obtain double-stranded oligos to anneal.
The ds oligos (50 μM stock) were diluted to a nal concentration of 5 nM by performing two 100-fold serial dilutions. First 100-fold dilution was accomplished with DNase/RNase-free water and then second100-fold dilution was completed with 1X Oligo annealing buffer supplied with the kit. These diluted ds oligos were used for cloning into the pENTR /U6 Entry Vector (Figure.S1). The annealed shRNA oligo nucleotides were ligated into RNAi Ready pENTR™ U6 vector by preparing 20 μl of ligation mixture containing 5X Ligation Buffer -4 μl, pENTRTM /U6 (0.5ng/ìL) -2 μl, ds oligo (5 nM; 1:10000 dilution) -1 μl, T4 DNA Ligase (1 U/μL) -1 μl and DNAse/RNase free water -12 μl. This ligation mixture was mixed well and incubated for 30 minutes at room temperature. Scrambled shRNA oligos (lacZ) supplied with the kit were also ligated to pENTR™/U6 vector and used as a negative control in the experiment.
Recombinant vector containing the shRNA molecules were transformed into the One Shot® TOP10 chemically competent E.coli cells by giving heat-shock (42°C for 30 seconds and then, immediately transferred to ice). Transformed cells were grown in 250μl of super optimal broth with catabolite repression (S.O.C.) medium at room temperature by incubating in horizontal shaker incubator (200 RPM) at 37°C for 1 hour. 200 μL of transformation mixture was spread on a pre-warmed LB agar plate containing 50 μg/ml of kanamycin and incubated overnight at 37°C.
Kanamycin resistant colonies were picked and inoculated into the LB broth containing kanamycin 50 µg/mL of media and incubated over night at 37°C to analyze the positive clones. Kanamycin resistant clones were screened for the presence of shRNA in the plasmid constructs by performing colony PCR with U6 forward primer: 5'GGACTATCATA TGCTTACCG3' and M13 reverse primer: 5'-CAGGAAACAGCTATGAC-3' by using 2.5 µL of 10XPCR buffer, 0.5 µL of dNTP mix (2.5 mM), 1.5 µL (30 ng) each of forward and reverse primers, 0.2 µL (1U) of Taq DNA polymerase, 2 µL of colony lysate and nuclease free water to make the volume up to 25 µL in 0.2 ml PCR tubes. Thermal cycling conditions followed were, initial denaturation at 95°C for 10 minutes followed by 35 cycles of denaturation at 95°C for 30seconds, primer annealing of 54°C for 30 seconds and extension at 72°C for 30 seconds and nal extension of 72°C for 10 minutes.
Plasmids containing shRNA molecules against SCD and SREBP1 genes were isolated by using Gene JET Plasmid Miniprep Kit (#K0503, Thermo Scienti c, USA). The plasmid obtained from each pENTR™/U6 entry construct was sequenced to con rm the sequence and correct orientation of the ds oligo insert.
Establishment of primary chicken embryonic hepatocyte (CEH) culture: Primary chicken embryonic hepatocyte culture was established by using 12-13 days old embryos. Embryos were collected aseptically by breaking the broad end of the egg after piercing the chorio-allantoic membrane (CAM) and placed in 9 cm petri-dish containing sterile Phosphate Buffer Saline (PBS) and rinsed thoroughly. Head, limbs and wings were separated and then, ventral side of the embryo was cut opened for collecting the liver lobes into the petri-dish containing the PBS. After thorough washing in PBS and mincing, liver tissues were transferred into the beaker containing sterile magnetic bar and approximately 10-15 ml of 0.125% trypsin. The beaker was placed on the magnetic stirrer for stirring at 37°C at about 100 RPM for less than 10 minutes. After allowing the pellets to settle down, supernatant was ltered through sterile double layered muslin cloth into a fresh beaker. The ltrate was centrifuged for 5 minutes at 3000 RPM and to stop the trypsin action, the resulting pellet was re-suspended in 5ml of growth medium (DMEM, Sigma) containing Fetal Bovine Serum (FBS). Now, the media with the liver cells were centrifuged at 3000 RPM for 3 min and then, the resulting pellet was re-suspended in 5ml of growth medium containing 10% FBS, 1% Tryptone Phosphate Broth and antibiotic-antimycotic solution. Haemocytometer was used to count the number of cells and accordingly, the cell suspension was diluted to get a cell concentration of 1 x 10 6 cells/ml. Approximately 2 x 10 5 cells/ cm 2 were seeded into the 25 cm 2 tissue culture ask and incubated at 37°C with 5% CO 2 . Medium was changed at regular intervals to have good growth of cells and counter the depression of pH.
Transfection of shRNA constructs into CEH cells: All the shRNA recombinant plasmid constructs against SCD and SREBP1 genes were transfected into the chicken primary embryonic hepatocytes in order to assess the activity of shRNA molecules. Approximately 0.4 ml of the hepatocyte cell suspension and plasmid containing shRNA molecules against SCD and SREBP1 genes were taken into electroporation cuvette and mixed gently. Single square wave pulse was given at a voltage of 150 mV for pulse length of 10 milli second. Both, pENTR U6/lacZ shRNA and pcDNA 1.2/V5/lacZ reported plasmids were co-transfected in to CEH. The pcDNA 1.2/V5/lacZ reported plasmid is used as a positive control for RNAi response in CEH. For getting optimal results, we had used 6 fold more entry construct DNA than reporter plasmid during co-transfection. Immediately after transfection, approximately 200 µl of cell suspension transferred to each well of a well plate containing 1.8 ml of growth medium and incubated at 37°C with 5% CO 2 level.

RNA extraction and cDNA synthesis;
The cells were harvested after 48 hours of transfection and RNA was isolated for transient RNAi analysis. The total RNA was isolated from the hepatic cells grown after successful transfection of the plasmids containing shRNA molecules against both target genes as per the manufacturer's protocol using Trizol (Sigma). The RNA samples were treated with DNase I (Fermentas) for removal of possible genomic DNA contamination. cDNA was synthesized by using High-Capacity cDNA Reverse transcription Kit (Applied Biosystems, #4368814) in a nal volume of 20 µL containing 10X Reverse Transcription (RT) buffer (2 µL), 10X RT random primers (2 µL), 100 mM of 25X dNTP mix (0.8 µL), RNase inhibitor (1 µL), MultiScribeTM Reverse Transcriptase (1 µL), Nuclease-free H2O (3.2 µL) and 1 ng RNA (10 µL). Reverse transcription was carried out in thermocycler (Mastercycler, Eppendorf, Germany) following the manufacturer's instructions, which includes the incubation of reaction at 25°C for 10 minutes followed by 37°C for 2 hours and 85°C for 5 minutes. The resulted cDNA was stored at -20°C till further use.
Real time quantitative PCR: The mRNA expression levels of SCD, SREBP1 genes and immune response genes viz. Interferon-A (IFN-A) and Interferon-B (IFN-B) genes in the transfected hepatic cells were quanti ed by using thermal cycler Applied Biosystems® Step One Real-Time PCR (Life Technologies) with SYBR® Green JumpStart™ TaqReadyMix™. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was used as an internal control for normalizing different amounts of input RNA (Table.2). The realtime PCR was performed for each sample in duplicates. Ampli cation curves for both the target genes showed typical initiation phase, exponential and plateau phase, indicating the successful exponential ampli cation of the product ( Figure.S2). Melt curves of the ampli ed products displayed speci c single peak for SCD, SREBP1 and GAPDH genes, indicating speci c ampli cation and homogeneity of the PCR products. These products were run on 1.5% agarose gel and expected amplicons sizes were observed.
The gene quanti cation was expressed as "n-fold up/down regulation of transcription" in relation to a reference sample, called the calibrator (mock transfected hepatocytes). The expression of target gene was calibrated by that of the reference gene, GAPDH, at each time point and converted to the relative expression (fold of expression), as follows: Western Blotting for detection of SCD and SREBP1 in cell culture: For Western blotting, polyclonal antibodies against SCD and SREBP1 were prepared for which a 591 bp length of SCD cDNA and a 507 bp length of SREBP1 cDNA were ampli ed by designing speci c primers (SCD_Ab Forward: AAGCTTATGCACCACCATCACCATCATAATATCCTCATGAGCCTG; SCD_Ab Reverse: GGATCCAAACATGTGAGCGCTG; SREBP1_Ab Forward: AAGCTTATGCACCACCATCACCATCATCCTGACAGCACCGTGTC; SREBP1_Ab Reverse: GGATCCGTCTGCCTTGATGAAGTG). The forward primers of bothe cDNAs contained with 6X histidine tag and BamHI restriction enzyme while the reverse primers contained Hind III restriction enzyme. The PCR ampli cation was carried out in 200 µL PCR tubes containing 2.5 µL of 10X buffer, 1 µL of dNTP mix (2.5mM), 1.5 µL (30ng) each of forward and reverse primers, 0.3 µL (1.5U) of Taq DNA polymerase, 1 µL of genomic DNA and nuclease free water to make up the volume up to 25 µL. Thermal cycling conditions followed for both the genes were initial denaturation at 95 o C for 10 minutes followed by 35 cycles of denaturation at 95 o C for 30 seconds, primer annealing at 55 o C for 30 seconds and extension at 72 o C for 1 minute 30 sec and a nal extension of 72 o C for 10 minutes. The ampli ed product was gel eluted and puri ed by using QIA quick gel extraction kit (Qiagen).
The pAcGFP1-C1 expression vector and puri ed amplicons of cDNA of both the genes were digested with BamHI and Hind III restriction enzymes. After RE digestion, cDNA of SCD gene and SREBP1 gene were cloned separately into the pAcGFP1-C1 expression vector. The ligated products were transformed into DH5α E. coli competent cells. The positive clones were screened and identi ed by colony PCR, plasmid PCR and sequencing. The recombinant plasmids were isolated by using Gene JET plasmid miniprep kit (Thermo Scienti c, USA). The recombinant plasmids of SCD and SREBP1 genes were transfected into the CEH by using the Gene Pulser Xcell TM Electroporation system (Biorad). After transfection, hepatocytes were grown in growth medium for 48 hrs and then, harvested for isolation of proteins. From the cell lysates, both SCD and SREBP1 proteins tagged with 6x histidine were extracted by using the His-Spin Protein Miniprep TM kit (GCC Biotech, Kolkata, India).
The puri ed protein was mixed with Freund's Adjuvant (IFA) and the mixture was injected subcutaneously (s/c). Primary injection was given wit Freund's Complete Adjuvant (CFA) and then, booster injections were given with Incomplete Freund's Adjuvant (IFA). A total of 6 male Wistar rats (2 for SCD protein, 2 for SREBP1 protein and 2 as control) of 8 weeks age were included in immunization schedule throughout period. The detailed Immunization protocol for rat polyclonal antibody production was as shown in Table S1. The IgG was puri ed from hyper immune sera using IgG puri cation kit (Himedia).
A period of 48 h after transfection of chicken embryonic hepatocyte cells with shRNAs against SCD and SREBP1 genes, cell pallets were harvested. The cell pellets were washed in ice cold PBS solution and then cell lysate was prepared by adding cold cell lysis buffer (150 mMNaCl, 1% NP-40, and 50 mM Tris, pH 7.4) @ 1ml per 150 cm 2 ask, agitated constantly and centrifuged at a rate of 12,000 rpm, at 4 °C for 20 min. Equal volume of Laemmli 2X sample loading buffer (10% SDS, 0.025% Bromophenol blue and 1% DTT) added to the obtained cell lysate and boiled at 100 °C for 3 min. About 20 μl of digested samples containing approximately 15-20 μg of protein loaded into the wells of SDS-PAGE with discontinuous buffer system Tris-Glycine-SDS buffer, pH 8.3 to separate the protein mixture. After completion of electrophoresis, the gel containing protein was transferred on to the polyvinylidene uoride (PVDF) membrane in the presence of Tris-Glycine-Methanol Buffer. After careful transfer of the gel, the blotted PVDF was immersed in 3% BSA blocking buffer with primary antibody (1:1000 dilution in TBST) and incubated at 4ºC for overnight. Then the membrane was washed three times with the TBS Tween 20 (TBST) for 5 minutes for each wash. After washing, membrane was incubated with anti-rat IgG HRP conjugate (Sigma) diluted to 1:1000 in TBS Tween 20 buffer for 1.5 hours with constant agitation. After washings with the TBS Tween 20 (TBST), the PVDF membrane was incubated in DAB substrate solution for 5-30 minutes until the color development. Soon after the appearance of brown color, substrate solution was drained and the reaction was stopped by adding distilled water.
Sandwich ELISA for quantitation of SCD and SREBP1 in cell culture: IgGs speci c for SCD and SREBP1 have been pre-coated onto a 96-well plate (12 x 8 Well Strips) and blocked separately. Test samples (cell lysate obtained after transfecting the shRNA molecules into the hepatocytes) were added to the wells, incubated and removed. HRP detector antibodies speci c for SCD and SREBP1 were added, incubated and followed by washing. HRP-Peroxidase Conjugate was then added, incubated and unbound conjugate was washed away. An enzymatic reaction was produced through the addition of TMB substrate which is catalyzed by HRP generating a blue color product that changes yellow after adding acidic stop solution. The density of yellow coloration read by absorbance at 450 nm is quantitatively proportional to the amount of sample SCD and SREBP1 captured in well.

Statistical analysis:
The statistical analysis in the experiments was carried out using trial version of SPSS 20. Univariate General linear model with Tukey's HSD and DMRT as post-hoc test was used to study the signi cant difference between different shRNA groups due to the knock down effect of target genes. Data from representative experiments were presented as Mean ± SE for different samples with differences determined by least signi cant differences at 5% level (P < 0.05). The degree of association between the expression of different genes was calculated by Pearson correlation coe cient.

Results shRNA molecules:
A total of 5shRNA molecules for each of SCD and SREBP1 genes were designed and synthesized. Local secondary structures of target mRNA sequences of SCD and SREBP1 genes were predicted and they revealed that all the all the shRNA target mRNAs consist of stem and loop secondary structures ( Figure.   The predicted values (negative) of ΔG overall, ΔG duplex and ΔG break−target (disruption energy) for mRNA target regions of SCD and SREBP1 genes were within the desirable range (Table 3 & Table 4). In case of SCD gene target mRNA region, overall ΔG value was almost same for shRNA2, shRNA4 and shRNA5. ΔG duplex value was the highest for shRNA5 and lowest for shRNA1. ΔG break−target (disruption energy) was highest for shRNA3, lowest for shRNA2. For SREBP1 gene mRNA target regions, overall ΔG value was highest for shRNA1, lowest for shRNA2. ΔG duplex value was highest for shRNA4 and lowest for shRNA1. ΔG break−target (disruption energy) was highest for shRNA2 and lowest for shRNA1. GC content of the shRNA target mRNA regions of SCD gene ranged from 43% to 52%, whereas, GC content of the shRNA target mRNA regions of SREBP1 gene ranged from 48% to 52% (Table 3 & Table 4).
A volume of 5 µl of 500 nM stock of ds oligos from each of the shRNA molecule was loaded on to the 4% agarose gel for checking the integrity of the annealed ds oligos ( Figure.S3 & Figure.S4). Annealed ds oligos were observed at 50 bp length, unannealed single-stranded oligos were observed at 25 bp length as the agarose gel is non-denaturing. Therefore, the single-stranded oligos do not resolve at the expected size due to formation of secondary structure.
Successfully annealed ds oligos were used further for cloning into the pENTR /U6 Entry Vector.
RNAi cassettes for production of shRNAs: Successfully annealed ds oligos were used for cloning into RNAi-Ready pENTR/U6 entry vector. After successful ligation of annealed ds oligos into RNAi-Ready pENTR/U6 entry vector, recombinant clones were transformed into competent E. coli cells. Recombinant clones were con rmed by colony PCR ( Figure.  The hepatocytes were transfected with anti-SCD and -SREBP1 shRNA constructs separately and grown in DMEM medium. The cells were harvested and then, total RNA and DNA were isolated from the harvested hepatocytes. For con rmation of successful transfection of recombinant shRNA molecules, DNA extracted from the harvested hepatocytes was used as template for performing the PCR with speci c primers for U6 Entry vector. The PCR products found at a product length of 287 bp con rmed the successful transfection of the recombinant shRNA constructs into the hepatocytes ( Figure.S9 & Figure.S10). The total RNA isolated from the hepatocytes into which recombinant shRNA constructs were transfected, was used for the synthesis of rst strand of cDNA. The cDNA was used for performing Real time PCR for assessing the expression of SCD, SREBP1and GAPDH genes in the hepatocytes after transfecting with recombinant shRNA constructs.
Silencing e ciency of anti-SCD and -SREBP1 shRNA constructs A signi cant (P<0.05) reduction was observed at the mRNA expression levels of SCD and SREBP1 genes, after transfecting the recombinant shRNA constructs into the hepatocytes, when compared with mock control. Expression levels (40-ΔCt) of SCD gene after transfecting with different shRNA constructs was 37.97, 37.61, 38.76, 38.42, 39.235 and 39.565 in the cells with shRNA1, shRNA2, shRNA3, shRNA4, shRNA5 and mock control, respectively. Expression of SCD gene in the transfected hepatocytes with shRNA1 and shRNA2 molecules reduced signi cantly (P<0.05) when compared with the mock control. Knock down e ciency of shRNA1, shRNA2, shRNA3, shRNA4 and shRNA5 molecules were 66.90%, 74.21%, 42.76%, 54.78% and 20.45%, respectively ( Figure.9). The shRNA2 molecule was found as the best molecule among all the shRNA molecules used for silencing the SCD gene expression in vitro.

Discussion
RNA interference (RNAi) has become one of the powerful tools in recent times to suppress the expression of speci c gene of interest [17][18], as a therapeutic option in disease management [19][20][21][22], as cancer treatment [23][24][25] and for understanding regulatory function of a gene by dsRNA molecules [26]. Suppression of gene expression achieved by cleavage of dsRNA molecules by ribonuclease protein (Dicer) followed by loading of siRNA molecules into RISC (RNA induced silencing complex) and then, guide strand of the siRNA molecule would guide the RISC towards the target mRNA sequence complementary with the siRNA for cleavage of target mRNA [27][28][29][30][31]. For successful reduction of gene expression through RNAi, designing of shRNA molecules against target gene plays crucial role [32]. In our study, anti shRNA molecules against SCD and SREBP1 genes were synthesized following the criteria such as moderate to low GC content, low internal stability of sense strand at 3' end (at least 3 bases of A/U at 15-19 positions of sense strand) and lack of internal repeats [33]. Basic Local Alignment Search Tool (BLAST) was used to detect whether our designed shRNA molecules are having the homology with any other genes to avoid the potential off-target effects.
Earlier studies revealed that the formation of secondary structures in the anti-sense/guide strand of shRNA molecule had critical role in determining the silencing e ciency and stated that a strong inverse correlation was observed between the degree of formation guide-RNA secondary structure formation and gene silencing e ciency of shRNA [34]. Patzel et al. 2005 classi ed and reported the secondary structures of guide strands of shRNA as: best silencing exhibited by sequences without any secondary structures, second best were stem loop structures with ≥ 2 free nucleotides at 5' and ≥ 4 free nucleotides at 3' end, next best were sequences having internal loop and two stem loop structures followed by stem loop structures with short free ends [34]. In accordance with the previous ndings in our study, the shRNA molecules with best silencing e ciency (i,e shRNA2 molecule for SCD gene and shRNA1 molecule for SREBP1 gene) did not posses any secondary structure in their anti-sense/guide strands. Second best shRNA molecules (i.e shRNA1 molecule for SCD gene and shRNA3 molecule for SREBP1 gene) have loop structures with ≥ 2 free nucleotides at 5' and ≥ 4 free nucleotides at 3' end. Some of the previous studies in chicken also reported that shRNA molecules having no secondary structures in their anti-sense/guide strand had expressed high knock down e ciency in other genes such as myostatin gene and ActRIIB gene [35][36]32].
Earlier studies suggested that the local structure of mRNA is one of the key factors and had a strong effect on the silencing e ciency of the shRNA molecule [37][38][39][40][41][42]. All the mRNA target regions of anti-SCD and anti-SREBP1 shRNA molecules revealed stem loop structures. Some studies inferred that presence of loop structures in the target mRNA regions provide easy access to the guide strand to bind with target region and had positive correlation with gene silencing e ciency [41,43]. Presence of paired nucleotides and hairpins in the secondary structure of target mRNA region likely to have negative effect on shRNA silencing [39,41]. In our study even though all target mRNA regions of SCD and SREBP1 genes have accessibility, the differences in the silencing e ciency might be because of other responsible factor. GC content of the target mRNA region have the crucial role in determining the silencing e ciency and it in uences the loading of shRNA molecule into RISC and target a nity and speci city [16]. A high GC content in shRNA molecule hinders the dissociation of siRNA duplex inhibiting RISC loading [44][45]. In contrary to that, some reports stated that a low GC content may reduce silencing e ciency by low target a nity and speci city [46]. The GC content of target mRNA region of our target genes ranged from 43-52%. Previous studies also suggested that GC content between 30% and 55% had positive correlation with silencing [33,47,48].
The shRNA molecules possessing high knock down e ciency must have higher negative value of ΔG overall, ΔG duplex and lesser negative value of ΔG break-target (disruption energy) [18,32]. Some studies postulated that shRNA molecules having the values of ΔG overall, ΔG duplex and ΔG break-target (disruption energy) within the range of − 25 to -35 kcal/mol, − 30 to − 40 kcal/mol and − 0.5 to − 1.5 kcal/mol exhibited good silencing e ciency [32,49]. In our study, all the shRNA molecules designed against SCD and SREBP1 mRNA possessing the values of ΔG overall, ΔG duplex and ΔG break-target (disruption energy) within the desirable range have shown good silencing e ciency. The shRNA2 molecule found as the best in silencing the expression of SCD mRNA having higher negative value of ΔG overall, ΔG duplex and lesser negative value of ΔG break-target (disruption energy) among all shRNAs designed. Similar ndings were observed in case of shRNA1 molecule, which was found as the best one in silencing the expression of SREBP1 gene expression. Possessing the thermodynamic properties within the desirable range might be the one of the reasons for better silencing e ciency of shRNA2 in SCD and shRNA1 in SREBP1 genes.
Silencing of the genes which are involved in the fatty acid biosynthesis has bene ts like production of lean meat and egg and reduction of fat deposition in several organs of the birds. Silencing of target genes which are involved in the fat biosynthesis by using RNA interference in chicken primary hepatocyte culture helped in the devising suitable in vitro models for knock down of the target gene. This technique can be used further for production of the knock down chicken with reduced fat production. Till date, there were no reports available in the literature regarding the silencing of fatty acid synthesis genes in chicken.
In the present study, the shRNA molecules were successfully transfected into the chicken hepatic cells and signi cant down regulation of the target genes were observed. These results suggest that the chicken embryo hepatocytes can be used for in vitro model for various functional studies. Signi cant reduction in the expression of SCD and SREBP1 genes in hepatocytes was observed after transfecting the shRNA molecules into them. The shRNA constructs against SCD gene showed the knock down e ciency ranged from 20.4-74.2%. In case of shRNA constructs against SREBP1 gene, they showed knock down e ciency ranging from 26.8-95.8%. The reasons behind the high knock down e ciency of shRNA2 against SCD gene and shRNA1 molecule against SREBP1 gene might be lack of secondary structures in their anti-sense/guide strand, desirable GC percentage (43%-52%) in sense strand, more accessibility in the target mRNA region i.e stem loop structures in secondary structure of mRNA region and possessing thermodynamic properties falling in desirable range. The expression of the SCD and SREBP1 examined in the cell lysates showed lower expression in the cells transfected with shRNAs indicating the similar trends as of mRNA expression in the cell culture. Thus, these results clearly demonstrated the successful down regulation of the gene expression by designed shRNA molecules against both the target genes under in vitro condition. In previous reports, by using shRNA molecules in Myostatin gene [50], ACTRIIA gene [51] and ACTRIIB gene [32] knock down e ciency of 68%, 87% and 82% were found respectively in chicken embryo broblasts cells. In duck embryo broblasts, different shRNA molecules mediated through lenti-virus have down regulated the MSTN gene expression by 61.6, 76.9 and 79.1%, respectively, when compared with control cells [52]. In caprine foetal broblasts, mRNA expression levels of myostatin gene were down regulated by 89% [53] and 72% [54] after transient transfection of shRNA molecules into them.
RNA interference is powerful technique for down regulating the particular gene but it may cause the activation of immune response genes under in vitro and in vivo condition. According to the recent reports, both immune cells and non-immune cells can detect the shRNA molecules irrespective of their sequence and leads to the stimulation of interferon and in ammatory cytokines under in vivo and in vitro conditions [55,56]. It was also reported that, shRNA molecules shorter than size of 30 bp can escape the PKR activation. It is known that the IFN response caused by activation of protein kinase R (PKR), ultimately leads to the inhibition of protein synthesis [56]. In the present study, we observed the expression of immune response genes particularly IFNA and IFNB in both tranfected and non-transfected control cells in case of both SCD and SREBP1 experiments. In both the genes, the non-signi cant differences of expression of IFNA and IFNB genes between shRNA transfecetd cells and either scrambled shRNA transfected cells or un-transfected negative control embryonic hepatocyte cells. In supporting to the results obtained in the present study, Patel et al. (2014) and Guru Vishnu et al. (2019) in ACTIIB gene in goat and chicken broblast cells, respectively [57,32]. We suggest that shRNAs designed in our study against both SCD and SREBP1 genes had the potential to be excellent shRNA molecules for further development of knock down chicken as the shRNA molecules having very good knock down e ciency escaped the interferon system of the cells.

Conclusion
It is concluded that short hairpin RNA-based silencing of SCD and SREBP1 genes under in vitro chicken hepatic cell culture system have been developed to minimize expression of these genes. The knock-down e ciency for both the genes reached more than 70% under in vitro cell culture system without affecting expression of immune response genes.

Declarations
Bold upper case letters -sense target sequence, Lower case letter -anti sense sequence, Italic letters -Loop          Knock down e ciencies of shRNA molecules of SCD gene and their associated ELISA titres in transfected and control chicken embryo hepatic cells.